Optical measuring systems of this type are being used for various measuring tasks like e.g. for length measuring or spectroscopy. In measuring systems of this type typically a laser diode is used as a light emitter and a suitable photo diode is used as a light detector. Laser absorption spectroscopy is used for example for gas detection. Thus, the main light beam emitted by the light emitter is detected by the light detector after passing through a gas or gas mix and a received signal is provided to a signal analyzer, in particular a lock in amplifier for evaluation. The signal analyzer separates constant interference patterns from the measuring signal of the light detector. However, the signal analyzer cannot completely eliminate time variable interference patterns from the received signal so that the detection sensitivity for the gas to be detected is significantly reduced due to the increased noise. Among other influences temperature influences trigger time variable interference patterns wherein the temperature influences change a length of an optical path for the main light beam from the light emitter to the light detector. Furthermore reflections and/or scattering of the main light beam at inner surfaces of the housing of the measuring system or at boundary surfaces of beam forming and/or beam directing optical or opto-mechanical components arranged in the housing like e.g. lenses or mirrors or at an inner or outer surface of the housing window can cause interference patterns of this type. The reflections or the scatterings can cause scatter light beams that are directed to the light emitter and/or the light detector. Scatter light beams of this type cause self-mixing in the light emitter and/or etalons at the light detector in that they interfere with the main light beam forming interference patterns and in that they form an interfered main light beam which is received by the light detector. These interference patterns are also a function of temperature and can therefore change over time. Self-mixing and etalons are caused by different length optical paths of the scatter light beams from the optical and/or opto-mechanical component respectively partially reflecting the main light beam up to the light emitter or the light detector relative to the optical path length of the main light beam. For the etalons a distance of the reflecting or scattering optical and/or opto-mechanical component up to the light detector is relevant, for the self-mixing the distance of the respective component to the aperture of the light emitter is relevant, which component forms a “receiver” for the scatter light beam entering through the aperture into the resonator and which interferes with the main beam that is partially reflected back at an inside of the laser.
In laser absorption spectroscopy for gas detection often a wave length modulation method is used. Thus, the wave length and typically also the intensity of the main light beam of the light emitter, for example of a continuously tunable diode laser is modulated with a frequency f wherein the wave length is varied over a possible absorption spectrum of a sample to be analyzed. The laser light is absorbed by the gas sample when the wave length of the light corresponds to a resonance frequency of the gas or when it is varied relative to the resonance frequency. When the main light beam after passing the gas sample impacts the light detector, e.g. a photo diode, the output signal of the light detector includes AC voltage components at the modulation frequency f and at the superimposing higher harmonic frequencies mf wherein m is a natural positive number. The demodulation of the output signal of the light detector at a harmonic frequency mf moves the measurement to a higher frequency band mf with lower 1/f noise which can improve measuring sensitivity of the optical measuring system.
When using lasers as light emitters due to the relatively large coherence length in particular an occurrence of interferences between the main light beam originating from the light emitter and of scatter light beams which are generated by undesirable reflections or scattering of the main light beam is particularly disadvantageous. Thus two different phenomena have to be differentiated. The phenomenon of influencing light emission by radiation that is back coupled into the laser aperture of the laser and which is known as self-mixing has a particularly strong effect upon the measurement since the back coupled radiation is amplified in the laser. In practical applications the main cause for the self-mixing is often found to be the scattering or reflection of the main light beam at the housing window which is provided for an exit of the main light beam and which protects the laser light emitter from environmental impacts like e.g. contamination or ambient humidity. Through the scattering this phenomenon is also present for inclined housing windows since a portion of the light can be directly or indirectly, for example through scattering at the housing inner wall, back coupled into the laser aperture. Other surfaces which are further remote from the aperture of the light emitter can also cause self-mixing, e.g. the reflection/scattering at the photo diode. Typically, however, their influence is lower. The second phenomenon that occurs in optical measuring systems is caused by interferences on the detector which are caused by different optical wave lengths of the main beam and the scatter beam and which are designated as etalons. Etalons like self-mixing are caused by scatter light beams at all optical and opto-mechanical components in the measuring system which then interfere with the main light beam at the detector. In order to minimize a falsification of the measuring signal of the light detector and thus of the measuring values by self-mixing and/or etalons either the self-mixing or the etalons themselves have to be reduced or their influence upon the measuring value has to be minimized.
In order to reduce these interferences various measures are known in the art. Among them are in particular rendering all housing windows non reflective, making the housing windows wedge shaped and a tilted, this means inclined arrangement of the housing windows in the beam path in order to prevent scattering as much as possible or in order to deflect the scattering out of the main light beam which scattering could cause interferences with the main light beam and thus self-mixing. Furthermore absorbing coatings in the housing of the optical measuring system and/or apertures arranged in the beam path can reduce reflections and scattering. In spite of these effective measures not all reflections and scatterings of the main light beam can be prevented which leads to self-mixing which typically has a very strong negative interference influence upon the measuring signal of the light detector in optical systems with coherent radiation. During operations the housing windows and surfaces of such optical measuring systems become contaminated quite frequently, e.g. by dust or condensation. This contamination greatly increases scattering of the main light beam and the interferences recited supra increase over the service life of the system. Thus, it is advantageous for the service life of such optical measuring systems to set the system up so that unavoidable contaminations affect the measuring signal of the system as little as possible.
When measuring spectra interferences recited supra are typically visible as periodic signals which are designated as “fringes” in the art. Self-mixing and etalons lead to a distortion of the measured gas absorption line for tunable diode laser spectroscopy systems (TDLS) which cannot be eliminated simply by calibration or computational methods. Thus, periodic interferences with a period in an order of magnitude of a width of the measured absorption line are particularly disadvantageous.
In principle the phenomenon self-mixing can be greatly reduced with an optical insulator. Unfortunately good quality insulators for non-telecom wave lengths are very expensive so that they are rarely used in industrial applications. It is furthermore appreciated that the optical insulator itself also has optical interfaces, wherein the optical interface of the optical insulator which optical interface is oriented towards the laser can also cause self-mixing by reflection/scattering. Persson proposes an “intensity referencing” method which reduces the effect of self-mixing while using balanced detection. However, this only achieves interference amplitude reductions by a factor of 10 (applied physics B87, 523-530 (2007)). Webster describes a very simple method to prevent etalons in that he inserts a coplanar tilted plate into a portion of the optical path wherein the coplanar plate is transparent for a laser wave length. Periodic pivoting of the tilted plate during the measurement facilitates that the interfering signal is averaged out due to the periodically changing optical path length. This solution helped to reduce the interfering signal by a factor of 30 (Opt. Soc. Am. B2 1464 (1985)). In order to prevent wear of mechanically moved components which leads to an increase in service life of the sensor piezo based actuators can be used. These, however, only facilitate generating small path length differences, wherein these elements are only effectively useable for averaging interferences with a very small free spectral range compared to a gas absorption line width. Silver and Stanton use for example a piezo electrical transducer (U.S. Pat. No. 4,934,816) which varies a longitudinal deflection of the mirror of a multi path cell and thus averages out the interferences based on the mirror arrangement. Piezo electrical transducers of this type, however, are typically rather expensive. Reid et alias have shown that it is possible by mixing an additional frequency into the modulation signal which frequency is slower than the modulation frequency, to average out such interferences also without changing the optical path lengths (Appl. Opt. 19, 3349-3354, 1980)). This principle, however, is only useable for interferences with a very small free spectral range since averaging has to be performed over at least one period. Almost the same result, however, can be achieved through low pass filtering, e.g. by using signal averaging in the data processing. Thus, the free spectral range has to differ significantly from a width of the gas absorption line. Otherwise also the absorption signal is influenced during the averaging. The preceding solutions are typically only useable specifically for a particular problem in an efficient manner. For example an optical insulator in front of the laser will only provide suppression of self-mixing but not suppression of etalons on the detector. Most of the cited solution proposals aim to reduce interferences in an averaged measuring signal of the detector.